Chemical looping system

ABSTRACT

A chemical looping system and a method of transferring oxygen therein are provided. The system has an air reactor adapted to receive air for oxidizing an oxygen carrier, a fuel reactor adapted to receive a fuel and the oxidized oxygen carrier for at least partially oxidizing the fuel by reducing the oxygen carrier to produce a gas. The oxygen carrier has oxide-dispersion-strengthened alloy particles.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2010/061200 filed Aug. 2, 2010 and claims the benefit thereof. The application is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to a chemical looping system and a method of transferring oxygen between therein.

BACKGROUND OF THE INVENTION

Chemical looping is a combustion technology with inherent separation of greenhouse gas CO₂. The technique involves the use of a metal oxide as an oxygen carrier for transferring oxygen from the air reactor to the fuel reactor. Thus, direct contact between fuel and air is avoided. The output product of oxidation of fuel, i.e., carbon dioxide, is kept separate from the rest of the flue gases, such as nitrogen and any un-reacted oxygen. Two reactors, i.e., the air reactor and the fuel reactor having interconnected fluidized beds are used for this process. The metal is oxidized to metal oxide with air in the air reactor and the oxidized metal oxide is reduced to metal in the fuel reactor. The reduced metal is transported back to the air reactor from the fuel reactor. Alternatively, metal-oxides with different oxidation states can be used as oxygen carriers between the air and the fuel reactor. The outlet gas from the air reactor comprises N₂ and un-reacted O₂ if any. The outlet gas from the fuel reactor comprises CO₂ and H₂O which can be separate by condensation. The CO₂ being separate from the flue gas is sequestration ready without the requirement of additional amount of energy and additional expensive separation units.

Chemical looping system can be used for producing power by combusting a gaseous fuel, and the technique is referred to as chemical looping combustion (CLC). The system can also be used for producing hydrogen and the technique is referred to as chemical looping reforming (CLR). The CLC system is generally integrated into a combined cycle power process.

SUMMARY OF THE INVENTION

It is an object of the embodiments of the invention to reduce the rate of decrease of the active surface area of the oxygen carrier particles for redox reactions in a chemical looping system.

The above object is achieved by a chemical looping system and a method of transferring oxygen in a chemical looping system according to the independent claims.

The oxygen carrier comprising the oxide-dispersion-strengthened alloy particles is oxidized in the air reactor and transported to the fuel reactor. The fuel in the fuel reactor reacts with the oxidized oxygen carrier and is oxidized. The oxygen carrier is reduced and the reduced oxygen carrier is transported back to the air reactor, where they are oxidized again. Thus, the oxygen carriers are circulated between the air reactor and the fuel reactor for transferring oxygen from the air reactor to the fuel reactor. The oxygen carrier being oxide-dispersion-strengthened alloy particles are less prone to sintering, and thus, more resistance to agglomeration during the high operating temperature of the chemical looping system. As the oxygen carrier particles are more resistant to agglomeration, the rate of decrease of the available active surface area for the oxidation/reduction reactions is reduced and thus, improving the redox activity over time. This enables the oxygen carriers to achieve longer operation life and reduce the operation costs of the chemical looping system.

According to an embodiment, the oxide-dispersion-strengthened alloy particles are composed of a metal having a dispersion of a metal oxide or a carbide. Dispersion of the metal oxide or the carbide into the metal enables in strengthening the metal and increase the redox activity. In conventional systems, the oxygen carrier is prepared by using binders such as alumina, silica, etc. In this case, the oxygen carrier is generally composed of a metal which can be oxidized to form a metal oxide to provide the oxygen for the combustion process, and an inert element as a binder for increasing the mechanical strength. Alternatively, the metal particles can be impregnated with a substrate, such as, a porous alumina substrate. In both cases, the target material performance with respect to strength and redox activity is not achieved.

According to yet another embodiment, the metal is selected from the group consisting of nickel, copper, iron, cobalt, manganese. The metals have relatively good oxygen transfer capabilities.

According to yet another embodiment, wherein the metal oxide is selected from the group consisting of cerium oxide, titanium oxide, and zirconium oxide.

According to yet another embodiment, wherein the carbide is silicon carbide.

According to yet another embodiment, wherein the metal oxide or the carbide is doped.

According to yet another embodiment, wherein the fuel comprises a carbonaceous fuel. The carbonaceous fuel can be combusted easily.

According to yet another embodiment, the fuel reactor is adapted to combust the fuel to produce the gas. The fuel is oxidized for combustion by reducing the oxygen carrier. The reduced oxygen carrier can be transported to the air reactor for oxidation, which is an exothermic reaction, thus producing energy.

According to yet another embodiment, the gas comprises CO₂ and H₂O. The CO₂ from the gas can easily be separated by condensing H₂O. Thus, the CO₂ obtained is sequestration ready as the same is separate from the flue gases. The CO₂ is separated from the flue gases without the requirement of additional amount of energy and additional expensive separation units.

According to yet another embodiment, wherein the fuel reactor is adapted to partially oxidize the fuel to produce the gas, wherein the gas comprises a reformer gas. The fuel is partially oxidized by reducing the oxygen carrier. The reduced oxygen carrier can be transported to the air reactor for oxidation.

According to yet another embodiment, wherein the reformer gas comprises H₂, CO, C₂O and H₂O. The H₂ of the reformer gas can be used as a fuel. Additional H₂ can be obtained by reacting CO and H₂O in a shift reactor. The CO₂ can easily be separated from H₂, and the separated CO₂ is sequestration ready as the same is separate from the flue gases. The CO₂ is separated from the flue gases without the requirement of additional amount of energy and additional expensive separation units.

According to yet another embodiment, wherein fuel reactor is further adapted to receive steam. The generation of H₂ can be enhanced by supplying steam into the fuel reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG. 1 illustrates a schematic block diagram of a chemical looping system according to an embodiment herein,

FIG. 2 illustrates an enlarged view of an ODS alloy particle according to an embodiment herein, and

FIG. 3 is a flow diagram illustrating a method of transferring oxygen in a chemical looping system according to an embodiment herein.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practiced without these specific details.

FIG. 1 illustrates a schematic block diagram of a chemical looping system according to an embodiment herein. As illustrated in the example of FIG. 1, the chemical lopping system 10 comprises an air reactor 15 and a fuel reactor 20. Typically, the air reactor 15 and the fuel reactor 20 are fluidized bed reactors. In the present example of FIG. 1, air is supplied as oxidant is into the air reactor 15, as designated by the arrow 25. A fuel is supplied into the fuel reactor 20, as designated by the arrow 30. The air reactor 15 and the fuel reactor are isolated and thus, there is no direct contact between air and fuel. Oxygen from the air reactor 15 is transferred to the fuel reactor 20 by circulating an oxygen carrier between the air reactor 15 and the fuel reactor 20, as designated by the arrows 35 and 40 respectively. The oxygen carrier is oxidized in the air reactor 15 forming an oxide. The oxide is then transported to the fuel reactor 20 where the fuel reduces the oxide to its original state. The oxygen carrier in its original state is transported back to the air reactor 15, where it is again oxidized and is transported to the fuel reactor 20. This transportation of the oxide to the fuel reactor 20 from the air reactor 15 and the transportation of the oxygen carrier in its original state from the fuel reactor 20 to the air reactor 15 is the circulation of the oxygen carrier between the air reactor 15 and fuel reactor 20. The air reactor 15 and the fuel reactor 20 are isolated and thus, there is no direct contact between air and fuel. The oxygen carrier transported from the air reactor 15 to the fuel reactor 20 provides the necessary oxygen required for the oxidation of the fuel in the fuel reactor 20.

According to an aspect, the oxygen carrier comprises oxide-dispersion-strengthened (ODS) alloy particles. The ODS alloy particles are composed of a metal having a dispersion of a metal oxide or a carbide. The metal particles are strengthened by the dispersion of the metal oxide or the carbide. The ODS alloy particles transfer oxygen from the air reactor 15 to the fuel reactor 20. The ODS alloy particles are in powder form, and thus, the particles are not grouped together. The ungrouped ODS alloy particles provide larger surface area for the redox reactions in the air reactor 15 and the fuel reactor 20. The metal used for preparing the ODS alloy particles include, but not limited to, nickel, copper, iron, cobalt, manganese, cadmium, and the like. In aspects, where a metal oxide is used for forming the ODS alloy particles, the metal oxide may include, but not limited to, cerium oxide, titanium oxide, zirconium oxide and the like. In an aspect, the carbide, may include, but not limited to, silicon carbide and tungsten carbide. The metal oxide and the carbide may be doped or un-doped. In an aspect, the ODS alloy particles may be formed by dispersing the metal oxide or the carbide into the metal by mechanical alloying. In an aspect, advantageously the ODS alloy particles may be supported on alumina, titanium oxide, YSR particles or other ceramics. The ODS alloy particles could also be recycled after the operation via thermal treatment to separate the metal and the metal oxide dispersion.

Referring still to FIG. 1, as only oxygen present in air is transported to the fuel reactor 20, the gas exiting the air reactor 15, as designated by the arrow 42, will comprise nitrogen and un-reacted oxygen if any. The gas exited from the air reactor 15 can be discharged into the atmosphere causing minimal or no CO₂ pollution. The gas produced due to the oxidation of the fuel by the oxygen carried in the fuel reactor 20 is exited from the reactor 20, as shown by the arrow 44.

Referring still to FIG. 1, in an aspect, the chemical looping system 10 can be operated as a chemical looping combustion (CLC) to produce energy by combusting the fuel. A carbonaceous fuel is supplied as the fuel into the fuel reactor 15. The term “carbonaceous fuel” hereinafter is referred to any material made of or containing carbon which is combustible or flammable. The carbonaceous fuel comprises, but not limited to, fossil fuels and fuels derived from fossil fuels. Advantageously, the carbonaceous fuel supplied into the fuel reactor 20 may be a gaseous fuel, such as, natural gas. Solid fuels can also used by gasifying the same to gaseous fuels and thereafter introducing the same into the fuel reactor 20. The gasification of the solid fuel may be performed in the fuel reactor 20 or may be performed externally in a separate reactor. In the present example, the carbonaceous fuel supplied into the fuel reactor 20 is methane. Air is supplied as the oxidant into the air reactor 15, as shown by the arrow 25. The metal present in the ODS alloy particles is oxidized by air in the air reactor 15 to form a metal oxide (M_(e)O). The ODS alloy particles containing the metal oxide are transported to the fuel reactor 20, as shown by the arrow 35. The oxidation of the ODS alloy particles is an exothermic reaction. As the chemical looping system 10 is operated as a CLC system, the fuel in the fuel reactor 20 is completely oxidized by reducing the metal oxide of the ODS alloy particles to metal. Thus, the fuel is combusted using the oxygen carried by the oxygen carrier from the air reactor 15. The reduction of the metal oxide of the ODS alloy particles to metal is an endothermic reaction. The ODS alloy particles containing the reduced metal are transported back to the air reactor 15, as shown by the arrow 40. In the present aspect, the gas stream exiting the fuel reactor 15, illustrated by the arrow 44 comprises CO₂ and H₂O. CO₂ can easily be separated from the exited gas stream by condensing H₂O. The separated CO₂ is pure as the same is separate from flue gases, and thus, ready for sequestration. This assists in separating CO₂ from N₂ and NO compounds without the consumption of additional energy and implementation of additional separation units.

The redox reactions in the air reactor 15 and the fuel reactor 20 can be summarized as follows:

Oxidation: exothermic

M_(e)+½O₂→M_(e)O  (1)

Reduction: endothermic

CH₄+4M_(e)O→CO₂+2H₂O+4M_(e)  (2)

Where M_(e) is metal, M_(e)O is metal oxide.

Referring still to FIG. 1, in another aspect, the chemical looping system 10 can be operated as a chemical looping reforming (CLR) to produce a reformer gas comprising H₂ by partially oxidizing the fuel. In accordance with this aspect, the fuel supplied into the fuel reactor 20 comprises a carbonaceous fuel. Advantageously, the carbonaceous fuel supplied into the fuel reactor 20 may be a natural gas. In the present example, the carbonaceous fuel supplied into the fuel reactor 20 is methane. In an aspect, to increase the yield of H₂, additional oxygen may be supplied into the fuel reactor 20 in the form of steam (H₂0). The steam may be supplied into the fuel reactor though the same inlet with which the fuel is supplied or may be supplied through a separate inlet. Air is supplied as the oxidant into the air reactor 15, as shown by the arrow 25. The metal present in the ODS alloy particles is oxidized by air in the air reactor 15 to form a metal oxide (MW). The ODS alloy particles containing the metal oxide are transported to the fuel reactor 20, as shown by the arrow 35. The oxidation of the ODS alloy particles is an exothermic reaction. The fuel in the fuel reactor 20 reacts with the metal oxide of the ODS alloy particles and is partially oxidized and the metal oxide is reduced to metal. The reduction of the metal oxide of the ODS alloy particles to metal is an endothermic reaction. The ODS alloy particles containing the reduced metal are transported back to the air reactor 15, as shown by the arrow 44. The partial oxidation of the fuel in the fuel reactor 20 produces a gas comprising a reformer gas. In an aspect, the reformer gas can comprise a syngas, CO₂ and H₂O. The syngas is a gas comprising a mixture of CO and H₂. In aspects where the reformer gas comprises a syngas, additional H₂ can be produced by reacting CO and H₂O in a subsequent shift reactor. CO₂ can easily be separated from the reformer gas. The separated CO₂ is pure as the same is separate from flue gases, and thus, ready for sequestration. This assists in separating CO₂ from N₂ and NO compounds without the consumption of additional energy and implementation of additional separation units.

The reactions in the air reactor 15, fuel reactor 20 and the shift reactor can be summarized as follows:

Oxidation: exothermic

M_(e)+½O₂→M_(e)O  (3)

Reduction: endothermic

2CH₄+4M_(e)O→CO₂+CO+H₂O+3M₂+4M₃  (4)

Shift reactor:

CO+H₂O→CO₂+H₂  (5)

Where M_(e) is metal, M_(e)O is metal oxide.

FIG. 2 illustrates an enlarged view of an ODS alloy particle according to an embodiment herein. In the example of FIG. 2, it is shown that the ODS alloy particle 45 is formed of a metal 50 and particles of a metal oxide 55 dispersed into the metal 26.

The ODS alloy particles 45 have increased strength relative to particles of simple metal. Using the ODS alloy particles 45 as oxygen carriers prevent sintering of the particles at the high operation temperature, and thus, prevent the decrease in the surface area per filling volume of the particles 45. Sintering of the metal-fuel particles leads to agglomeration of the particles during high temperature treatment, and thus, degradation in the performance of a chemical looping system with time, as the surface area per filling volume of the particles decreases. Thus, the degradation rate of the performance of the chemical looping system 10 of FIG. 1 with time is reduced and the life-time of the chemical looping system 10 is increased by using ODS alloy particles 45 as oxygen carriers as the same are less prone to sintering, and thus, more resistant to agglomeration. The ODS alloy particles being more resistant to agglomeration enable in reducing the rate of decrease of the active surface area for redox reactions at the fuel reactor 20 of FIG. 1 and the oxidation reactor 15 of FIG. 1 and also improve the redox activity over time of the oxygen carrier. Additionally, the ODS alloy particles can posses relatively higher ionic conductivity during redox processes if the dispersed oxide on the metal is an O₂ conductor. The higher ionic conductivity enables in enhancing the redox reaction rate.

FIG. 3, with reference to FIG. 1 through FIG. 2, is a flow diagram illustrating a method of transferring oxygen in a chemical looping system according to an embodiment herein. At block 60, an air reactor 15 adapted to receive an oxidant for oxidizing an oxygen carrier is provided. Next, at block 65, a fuel reactor 20 adapted to receive a fuel and the oxidized oxygen carrier for at least partially oxidizing the fuel by reducing the oxygen carrier to produce a gas, and wherein, the oxygen carrier comprises oxide-dispersion-strengthened alloy particles.

The embodiments described herein enable in increasing the efficiency of the chemical looping system. Moreover, the duration for which the oxygen carriers can be re-circulated within the chemical looping system is increased. Additionally, this enables in reducing the operating cost of the system.

While this invention has been described in detail with reference to certain preferred embodiments, it should be appreciated that the present invention is not limited to those precise embodiments. Rather, in view of the present disclosure which describes the current best mode for practicing the invention, many modifications and variations would present themselves, to those of skill in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope. 

1.-15. (canceled)
 16. A chemical looping system, comprising: an air reactor adapted to receive air for oxidizing an oxygen carrier, and a fuel reactor adapted to receive a fuel and the oxidized oxygen carrier for at least partially oxidizing the fuel by reducing the oxygen carrier to produce a gas, wherein the oxygen carrier comprises oxide-dispersion-strengthened alloy particles.
 17. The chemical looping system according to claim 16, wherein the oxide-dispersion-strengthened alloy particles are composed of a metal having a dispersion of a metal oxide or a carbide.
 18. The chemical looping system according to claim 17, wherein the metal is selected from the group consisting of: nickel, copper, iron, cobalt, manganese, and cadmium.
 19. The chemical looping system according to claim 17, wherein the metal oxide is selected from the group consisting of: cerium oxide, titanium oxide, and zirconium oxide.
 20. The chemical looping system according to claim 17, wherein the carbide is silicon carbide.
 21. The chemical looping system according to claim 17, wherein the metal oxide or the carbide is doped.
 22. The chemical looping system according to claim 16, wherein the fuel comprises a carbonaceous fuel.
 23. The chemical looping system according to claim 16, wherein the fuel reactor is adapted to combust the fuel to produce the gas.
 24. The chemical looping system according to claim 16, wherein the gas comprises CO₂ and H₂O.
 25. The chemical looping system according to claim 16, wherein the fuel reactor is adapted to partially oxidize the fuel to produce the gas, and wherein the gas comprises a reformer gas.
 26. The chemical looping system according to claim 25, wherein the reformer gas comprises H₂, CO, C₂O and H₂O.
 27. The chemical looping system according to claim 16, wherein fuel reactor is further adapted to receive a steam.
 28. A method for transferring an oxygen in a chemical looping system, comprising: providing an air reactor adapted to receive air for oxidizing an oxygen carrier, and providing a fuel reactor adapted to receive a fuel and the oxidized oxygen carrier for at least partially oxidizing the fuel by reducing the oxygen carrier to produce a gas, wherein the oxygen carrier comprises oxide-dispersion-strengthened alloy particles.
 29. The method according to claim 28, wherein the oxide-dispersion strengthened alloy particles are composed of a metal having a dispersion of a metal oxide or a carbide.
 30. The method according to claim 29, wherein the metal is selected from the group consisting of: nickel, copper, iron, cobalt, manganese, and cadmium. 